The novel expression of nonclassical MHC class I molecules by the primate trophoblast is considered to be biologically significant, yet the in vivo responses of the maternal physiological and immunological systems to these molecules at the maternal-fetal interface is difficult to investigate in human pregnancy. We have recently demonstrated the biological relevance of primate placental MHC class I expression with passive immunization of rhesus monkeys during early pregnancy. Treatment with a specific monoclonal antibody to a nonpolymorphic MHC class I molecule designated Mamu-AG expressed in the rhesus placenta, homologous to HLA-G expressed in the human placenta, resulted in delay or disruption of placental development and endometrial responses to implantation. These studies, however, were unable to determine which aspects were due to direct effects of Mamu-AG, which were due to a soluble Mamu-AG isoform, and which may have been due to other secondary alterations in placental function. We hypothesize that Mamu-AG interacts with decidual NK cells to promote appropriate responses in the early pregnancy decidua, including the differentiation and distribution of macrophages and T cells, the modification of smooth muscle in maternal vessels, and differentiation in the functional endometrium. To begin to address this hypothesis, we will evaluate soluble Mamu-AG expression and define its effects on rhesus monkey leukocytes and endometrial differentiation with three specific aims. Specific Aim 1. To define the effects of recombinant soluble Mamu-AG on the nonpregnant endometrium. Specific Aim 2. To define circulating soluble Mamu-AG in pregnant and in nonpregnant rhesus monkeys. Specific Aim 3. To determine the effects of soluble Mamu-AG on cytokine secretion, activation, apoptosis, and proliferation in NK cells, macrophages and T cells. With these experiments we will begin to understand the direct role of soluble MHC class I molecules in regulating specific functions of leukocyte subsets as well as stromal and vascular elements in the endometrium at the maternal-fetal interface. Placental-maternal immune interactions are hypothesized to contribute to pathological conditions in pregnancy, ranging from infertility and spontaneous miscarriage to preeclampsia. Yet, there is a dearth of experimental evidence in vivo to support these hypotheses. Our proposed studies could not be carried out in clinical human experiments, but the close similarities between human and nonhuman primate pregnancy will allow us to define the endometrial response to the novel placental MHC phenotype in early pregnancy, and conduct hypothesis-testing in vivo research with direct significance for human pregnancy. PUBLIC HEALTH RELEVANCE: Placental-maternal immune interactions are hypothesized to contribute to pathological conditions in pregnancy, ranging from infertility and spontaneous miscarriage to preeclampsia. Yet, there is a dearth of experimental evidence in vivo to support these hypotheses. Our proposed studies could not be carried out in clinical human experiments, but the close similarities between human and nonhuman primate pregnancy will allow us to define the endometrial response to placental MHC in early rhesus gestation and conduct hypothesis-testing in vivo research with direct significance for human pregnancy. A better understanding of the immune response to the establishment of pregnancy may also have significance not only for therapy of threatened pregnancies, but for graft acceptance and cancer immunotherapy as well, owing to recent considerations of HLA-G in transplantation and malignant transformation.